Key to the performance of MTJs (magnetic tunnel junctions) for MRAM (magnetic random access memory) and read heads are: (a) well-controlled magnetization of the pinned layer, large pinning field and high thermal stability, (b) integrity of the tunnel barrier layer, and (c) well-controlled magnetization and switching of the free layer.
For (a), the pinned layer of the MTJ device is typically a SyAF (synthetic antiferromagnetic) layer (e.g. AFM/CoFe/Ru/CoFe). Use of SyAF pinned layer in the MTJ structure not only improves thermal stability but also minimizes the interlayer coupling field (offset field) between the pinned layer and the free layer. For (b), the tunnel barrier commonly used is either a thin layer of amorphous AlOx or crystalline MgO. It has been shown, in the case of a NiFe free layer-MTJ made with amorphous AlOx and crystalline MgO barrier layers is capable of delivering dR/R (magnetoresistive change) more than 40% and 85%, respectively (1, 2). For (c), the free layer for the MRAM-MTJ is best made of a thin permalloy (NiFe) film for its reproducible and reliable switching characteristics (i.e. low switching field (Hc) and switching field uniformity σHc). It is important that the MRAM-MTJ free layer exhibit low magnetostriction λs (lambda <1×10−6).
The typical cap layer for a conventional MTJ stack is a nonmagnetic conductive metal such as Ta or TaN. The disadvantage of using a Ta capping layer is that, during thermal annealing, Ta diffuses into the NiFe free layer and not only reduces the free layer moment (Bs) but also makes the NiFe free layer very magnetostrictive with a λs>5×10−6 (1). In MRAM application, we found that a high percentage of free layer switching, in Ta capped NiFe-MTJs, is through a vortex structure which results in poor switching field uniformity. To eliminate NiFe/Ta inter-diffusion, the prior art has used Ru to cap the NiFe (free)-MTJ. However, dR/R of the Ru cap MTJ is severely degraded (from 40% to 30%). To reduce NiFe/Ta inter-diffusion while still preserving high dR/R, the NiFe (free)-MTJ can be capped with a Ru/Ta/Ru structure (1). During thermal annealing to fix the pinned layer magnetization direction, the intermediate Ta layer in the tri-layer cap is capable of gettering oxygen atoms in the underlying NiFe free layer. Consequently, the NiFe free layer is less oxygen contaminated and a sharper AlOx/NiFe interface is formed, resulting in an improved dR/R.
A routine search of the prior art was performed with the following references of interest being found:
U.S. Patent Applications 2006/0114716 and 2004/0085681 (Kai et al) teach a composite free layer comprising two layers of NiFe with a layer of Hf in between. U.S. Pat. No. 7,072,208 (Min et al—Headway) shows a free layer of NiFe with a dopant concentration of 1-40% by weight of Hf. U.S. Patent Application 2004/0257719 (Ohba et al) discloses a NiFe—Hf free layer.
U.S. Pat. No. 7,067,331 (Slaughter et al) describes a free layer of CoFeHf. U.S. Patent Application 2006/0119990 (Nishiyama et al) shows a NiFe free layer with Hf aggregated into the crystal grain boundary. U.S. Pat. No. 6,710,987 (Sun et al) discloses NiFe as a free layer alloyed with Cr, Ta, Mo, Nb, or Zr to have low magnetization.
U.S. Pat. No. 7,026,673 (Abraham) shows NiFe free layer alloyed with Ge, B, V, Mb, or Os. This patent teaches the use of “low magnetization materials” for high performance magnetic memory devices. Essentially, an MTJ is made with a thicker free layer of low magnetization materials to improve AQF (array quality factor, defined as Hc/σHc). For a permalloy (NiFe19%) Ms=800 emu/cc, the low magnetization material is preferably a NiFe alloy having Ms less than 600 emu/cc. The low magnetization material comprises a Ni—Fe alloy, including one or more moment reducing elements such as Ge, B, V, Mo, or Os and combinations. Among these elements, only V has an oxidation potential higher than Fe and Ni.
U.S. Pat. No. 6,903,909 (Sharma et al) teaches a NiFe pinned layer to which an amorphizing agent Hf is added. U.S. Patent Application 2006/0056114 (Fukumoto et al) describes a magnetic layer that can be NiFeHf. U.S. Patent Application 2002/0054462 (Sun et al) shows a free layer of NiFe/CoFe with a barrier layer thereover comprising an alloy of Ni and a Hf element.
Additional references of interest included:
    1. C. Horng et. al. 2003 Invention disclosure “A novel capping layer for forming high performance MTJ for MRAM application”.    2. R W. Dave et. al., “MgO-based tunnel junction material for high-speed toggle MRAM”, Abstract ED-05, 2005 MMM conference.    3. M. Nagamine et. al. “Conceptual material design for MTJ cap layer for high MR ratio”, abstract ED-10, 50th MMM conference.    4. C. Horng et. al. EMG06-005 “A novel method to form nonmagnetic-NiFeMg cap for the NiFe(free layer)-MTJ to enhance dR/R”.    5. C. Homg & R. Tong, EMG06-011 “A novel material to cap the NiFe(free layer)-MTJ to enhance dR/R and method of forming the cap structure”, and I-IMG06-16 “A novel material to cap the NiFe(free layer)-MgO-MTJ to enhance dR/R and method of forming the cap structure”.    6. M. Chen et. al. “Ternary NiFeX as soft biasing film in a magnetoresistive sensor”, J. Appl. Phys, 69. n 5631-33 (1991).    7. U.S. Pat. No. 7,026,673 “Low magnetization materials for high performance magnetic memory devices”.